1. Field of the Invention
The present invention relates to cooling systems for gas turbine engines. More particularly, the present invention pertains to a method and system for cooling an engine by utilizing air from the intermediate stage of a compressor to cool a flow path adjacent a combustor stage leading to a turbine stage. An improved seal prevents substantial leakage of compressor discharge (CD) air from entering the flow path. Since the air supplied by the intermediate stage compressor is at a lower temperature than CD air, less volume is needed for the same degree of cooling. Furthermore, since less work is performed on the air supplied by the intermediate stage compressor, fuel efficiency is improved and fuel consumption reduced.
2. Discussion of the Background
Air leakage through gas turbine engine seals may significantly increase fuel consumption, reduce engine efficiency and increase maintenance costs by increasing turbine inlet temperatures. Gas turbine engines have traditionally included labyrinth seals at critical sealing locations. Labyrinth seals control the leakage of high pressure gas, such as compressor discharge air, from a generally high pressure area to a generally low pressure area. The seals operate by throttling gas flow through a series of annular constrictions formed between annular teeth, which may be located on a rotating component, and an annular rub strip which may be located on a stationary engine member. The rub strips are abradable to allow the teeth to rub lightly during dynamic operating conditions, such as thermal transients or maneuver loads. The effectiveness of these labyrinth seals is dependent on keeping the radial clearance between the rub strip and teeth to a minimum.
However, the minimum radial clearance is limited by manufacturing tolerances, rotor concentricity control, and thermal and centrifugal growths between rotating and stationary components. Too small a radial clearance results in premature seal wear and possible engine damage, while too large a radial clearance results in excess leakage. As seal diameters and gas temperatures increase in advanced engines, the radial clearance of labyrinth seals has increased, thereby diminishing their effectiveness.
Gas bearing face seals provide an alternative to the labyrinth seal. U.S. Pat. No. 3,383,033 issued to C. Moore and assigned to the assignee of the present invention discloses a gas bearing face seal for use as a compressor discharge seal. Although the seal as disclosed may be an improvement over the labyrinth seal, the disclosed seal includes a number of potential disadvantages. For example, the disclosed seal does not include means for maintaining the seal concentric with respect to the axis of the engine or with other seal components. Test analysis has demonstrated that concentricity of a ring seal with respect to other seal components is important for good seal operation. Concentricity helps maintain concentric, balanced pressure forces on the seal components and promotes good sealing characteristics while allowing for the use of smaller, more lightweight seal components.
The seal disclosed in U.S. Pat. No. 3,383,033 uses a relatively massive ring member and therefore possesses a high mass inertia. Ideally, the ring member should have a low mass inertia and have a relatively high compliance for reduced seal weight and, more importantly, reduced hysteresis in the axial clearance between the radial sealing surface and the face of the rotor. A low mass inertia ring member can more quickly and efficiently track motion of the sealing surface with lower actuating forces.
It is also believed that the seal disclosed in U.S. Pat. No. 3,383,033 may include a propensity for the clearance between the seal and face of the rotating seal ring member to vary according to thermal growth and other factors causing changes in pressure forces and poor sealing. Also, the disclosed seal includes an auxiliary restrictor tooth which is integral with or mounted on the ring member which adds weight to the ring member requiring heavy spring means to be used to bias the ring member away from the sealing surface. Furthermore, it is believed that the disclosed seal does not efficiently vent air exiting the air bearing space and the restrictor tooth to the low pressure region. Improved venting is desired to assure proper pressure balance on the ring member.
The above disadvantages of the seal disclosed in U.S. Pat. No. 3,383,033 are addressed in the U.S. patent application entitled "Gas Bearing Sealing Means", filed Mar. 25, 1991, having U.S. Ser. No. 07/675,977, hereinafter referred to as the "'977 application" which is assigned to the assignee of the present invention, herein incorporated by reference in its entirety. For purposes of this application, the "Gas Bearing Sealing Means" disclosed in the application filed on Mar. 25, 1991 shall hereinafter be referred to as the "improved gas bearing seal".
The improved gas bearing seal maintains a concentric relationship between gas bearing face seal components to establish concentric pressure forces on the components. The improved gas bearing seal has a face seal ring member which is pressurized preferably in the radially outward direction in order to maintain ring shape. The improved gas bearing seal has a reduced mass inertia face seal member for reducing sealing clearance hysteresis. The improved gas bearing seal has means for maintaining a pressure balance even when the engine stationary structure is exposed to differential growth. Furthermore, the improved gas bearing face seal requires less tension in the spring means which biases the face seal member away from the sealing surface and achieves improved flow from a gas bearing face surface and a primary flow restrictor means.
The above features of the improved gas bearing seal are achieved by a seal assembly for restricting flow from a relatively higher pressure region to a relatively lower pressure region at a location which interfaces an engine stationary member and an engine rotating member. The seal assembly of the improved gas bearing seal includes the following, as noted in '977 application: a face seal ring member with a primary restrictor sealing dam, housing means including a housing structure fixed to an engine stationary structure, and a secondary seal means, which may be a piston ring seal, in sealing engagement with the housing means and the face seal ring member. The face seal ring member is mounted on the housing for movement of the primary sealing dam into a predetermined clearance with a primary sealing surface on the rotating engine member.
The assembly includes support means for supporting the face seal ring member concentrically with respect to a housing surface to maintain concentric sealing forces on seal components, while permitting radial growth between the face seal ring member and a housing structure. Control ring means concentrically positioned on the housing structure may be used in high temperature applications to maintain pressure balance on seal components during differential growth of the housing structure with respect to the face seal ring member. The seal assembly may be configured to utilize the high pressure region being sealed to pressurize the ring member radially outward, for maintaining ring roundness. Auxiliary restricting teeth can be mounted on a structure separate from the face seal member, the teeth being concentric with respect to a face seal member surface and a surface on the engine rotating member. Venting of the gas bearing space and primary restrictor flow is improved by including a vent channel turning vane and circumferentially angled vent passages to reduce the pressure drop from the vent channel to the low pressure region.
The improved gas bearing seal significantly reduces leakage of compressor discharge air so that more compressor discharge air is utilized by the combustor. The improved gas bearing seal also significantly reduces the temperature rise of the air that leaks through the seal. However, the superior sealing properties of the improved gas bearing seal do not allow enough air to leak through to a flow path which comprises a cavity formed by a metal structure so the air can be used to cool the metal structure and the turbine blades which are aerodynamically connected to the flow path. Thus, a need is seen for a method and assembly which can utilize the effective sealing properties of the improved gas bearing seal on compressor discharge air while providing adequate or improved cooling for the engine.